The Wood Ball Shell Jig

 I made this jig before attending the Winter Blast fireworks convention, and I did not have an actual hemi to fit it to. It worked very nicely, but I have thought of some improvements, and have incorporated them here. They involve refinement of the measurements, and a different method of resting on the edge of the shell.

Start with downloading the drawing which is actual size: Jig drawing Print two copies of it on your printer. One of the copies can be used to make patterns, and the other to line it all up while gluing. The pieces are cut out of 3/4 inch thick oak.

First, cut the bottom pieces as outlined in black. I used a band saw for this. Don’t smooth or sand anything yet.

Then, cut the next layer pieces, outlined in blue, and glue them to the bottom pieces in the same position as the picture. After the glue is set 24 hours, a band saw can cut the sides of the second layer to match the first layer. Then, the sides are sanded. The curved transition between the upper crosspiece and the bottom layer is cut with a band saw, but the final shaping and sanding of this is done after the crosspiece is glued.

The upper crosspiece is made, and #19 (0.166 inch diameter) holes and countersinks are drilled into it to later be able to attach the wood ball half with its connector, using #8 x 1.5 inch brass flat head wood screws. Then the crosspiece is glued to the lower pieces. If they are clamped onto a piece of scrap plywood over the printed diagram, you can make sure that they are lined up properly.

After the glue is all set for 24 hours, it can all be shaped and sanded: The crosspiece and the bottom pieces can be feathered into each other, and edges rounded. The screws (#8 x 2 inch brass FHWS) going through the ends of the crosspieces to the lower parts are placed after the gluing is all done. These screws are to reinforce it all. If you drill holes and try to put the screws in during the gluing process, it will be difficult to get it all lined up exactly. The glue may be strong enough to hold it without the screws, but I like to build stuff to last. Be sure to use brass screws, as steel screws carry the danger of creating sparks. Considering that this will all be used around explosives, sparks are a bad thing!

For the half wood ball, I used a wood cap for a railing or a fence post that I got from Home Depot. I cut the 2 15/16” diameter ball section in half, and then cut off the parts that didn’t look like a ball and rounded and sanded the ball half. A piece of 3/4” oak was cut to bridge between the crosspiece and the ball half, and glued to the ball half.

A 7/16” hole was drilled into the center of the ball to accommodate the passfire tube. This hole doesn’t cause any problem when packing the hemi that is without a passfire tube, since the tissue paper covers the hole for that half. Two more #26 (0.147”) holes are drilled into the wood connector so that the ball can be screwed to the jig.

If desired, more half wood balls can be made to attach to the crosspiece, for the possibility of making a triple petal shell. Your local craft store will likely have wood balls. A ball-half larger than the one I used can be made from a hemi for a 4 inch shell.

In the pictures here, I used pieces of sheet brass to rest on the edge of the hemi. With my first design I bent the edges of brass over to hook onto the edge of the hemi. I found out that the eye screws hold onto the hemi so well that just resting brass plates on the edge of the shell is easier to make, and there is nothing overhanging the edge of the hemi to possibly get in the way of the top row of stars. I fastened the plates with #4 x 3/8” stainless steel flat head sheet metal screws, but other screws could work as well. Just don’t use steel. Sheet brass 0.050” thickness is available at McMaster-Carr number 8956K445.

I finished the jig with two coats of polyurethane varnish.